No. 461.) NORTH AMERICAN INVERTEBRATES. 295 
sion. Each maxilla bears a jointed appendage, the palpus. In 
the female, the palpus is always simple, but in the male, when 
mature, the last joint is enlarged, concave within, and furnished 
with a number of more or less corneous and curved pieces, 
which serve as accessory sexual organs. The shape of these 
male palpi is of great value in the study of species. The 
sternum is the ventral plate of the cephalothorax; it is sur- 
rounded by the eight legs. In some species there are scars 
or impressions on the sternum. The legs are numbered from 
before backward as follows: I, II, III, IV. They are seven- 
jointed, each joint from the basal outward known as follows : 
coxa, trochanter, femur, pa- 
tella, tibia, metatarsus or pro- 
tarsus, and tarsus; in a few 
forms there is a small eighth 
joint, the onychium. At the 
tip of the tarsus, or onychium 
if present, are two claws, equal 
in size; below and between 
them, in some families is a 
third. chaw: Gomina M Her with two claws and brush, and with 
place of the third claw there 
is a dense brush or fascicle of hair. The claws are often toothed 
or pectinate. In a few groups there are specialized branched 
hairs at the tip of the tarsus which may act as accessory claws. 
Sometimes there is a brush of hairs along the lower side of the 
tarsus or metatarsus ; this is called a scopula. Sometimes there 
is a row of serrate bristles under tarsus IV ; this is known as a 



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om 
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comb. . 
The abdomen is joined to the cephalothorax by a slender ped- 
icel. At or near the tip of the abdomen on the 
under or ventral side are the spinning organs, 
or spinnerets. They are of three pairs, the mid- 
dle pair smaller and concealed by the other two. 
Sometimes one pair is very plainly of two joints. 
At the base of the lower pair there is sometimes 
a transverse surface provided with (Fig. 4) spinning tubes simi- 
lar to those of the spinnerets;; this is the cribellum. Comple- 

FıG. 4.— Cribellum. 
